Known is an inverted pendulum type vehicle or an omni-directional vehicle comprising a pair of drive assemblies individually actuated by electric motors and a main wheel held between the drive assemblies and frictionally driven by the drive assemblies. See WO2008132779A1(US20100096905A1) for instance. Each drive assembly comprises a drive disk coaxially opposing the drive disk of the other drive assembly and a plurality of drive rollers obliquely arranged along the circumference of the drive disk at a regular interval so as to be individually rotatable. The main wheel comprises a ring-shaped annular member rotatably supported by a frame around a central axial line thereof and a plurality of driven rollers arranged along the circumference of the annular member so as to be rotatable around the respective tangential lines. As the drive disks are turned by the electric motors, the driven rollers are frictionally driven by the drive rollers. When the drive rollers are turned around the tangential directions of the main wheel, the vehicle is driven in a lateral direction. When the main wheel is turned around the central axial line thereof, the vehicle is driven in a fore and aft direction. The direction of motion of the vehicle can be selected as desired by suitably adjusting the difference between the rotational speeds of the two drive disks.
However, according to this previously proposed inverted pendulum type vehicle, because the main wheel is simply held between the two drive assemblies, and no arrangement is made to positively position the main wheel with respect to the drive disks, the annular member and drive disks may not be coaxially disposed. Therefore, only a small portion of the drive rollers are allowed to engage the driven rollers, and the efficiency of transmitting power from the drive assemblies to the main wheel may not be high. Similarly, when the annular member is deformed from a true circular shape, a relatively large part of the drive rollers do not engage the corresponding driven rollers.